Technical Field
The present subject matter relates to wireless communication. More specifically, the present subject matter relates to a radio access network (RAN) using an adaptive fronthaul protocol.
Background Art
A Radio Access Network (RAN) provides access to a core network for a wireless terminal, such as a smartphone. RANs have progressed through several different generations of technology, and are sometimes referred to by a so-called “generation number,” such as 3G, or 4G networks. An example 2G RAN is the GSM (Global System for Mobile Communications) Radio Access Network (GRAN), example 3G RANs include the GSM EDGE Radio Access Network (GERAN), and the Universal Mobile Telecommunications System (UMTS). An example 4G network is the Long-Term Evolution Advanced (LTE-A) which is also known as the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) and may also be referred to simply as “LTE” herein. Each RAN communicates with wireless terminals using radio frequency communication protocols defined by the RAN at frequencies supported by the RAN and licensed by a particular communications company, or carrier. The frequencies are modulated using a variety of techniques, depending on the RAN, to carry digital information that can be used for voice transmission and/or data transfer.
Each RAN can define its own software structure, but many generally conform to the 7-layer Open Systems Interconnection (OSI) model. The seven layers include a lowest layer, layer 1, which is commonly referred to as the physical layer or PHY, which describes the transmission and reception of raw bit streams over a physical medium. The next layer, layer 2, is known as the data link layer, but often is referred to by the name of a protocol that resides at that layer, such as the medium access protocol (MAC), or point-to-point protocol (PPP), which provide for transmission of data frames between two nodes connected by a physical layer. The third layer, known as the network layer, manages a multi-node network and handles such issues as addressing, to send packets of data between nodes. The internet protocol (IP) is a commonly used network layer protocol. The next layer, layer 4, is known as the transport layer. Common transport protocols include the transmission control protocol (TCP) and the user datagram protocol (UDP). Transport protocols manage transmission of data segments between nodes of the network. Layer 5, the session layer, manages communication sessions, layer 6, the presentation layer, translates data between a networking service and an application, and the top layer, layer 7 or the application layer, provides high-level application programming interfaces (APIs) for network related services.
More details of an E-UTRAN are provided as an example. The specifications for E-UTRAN are developed and published by the 3rd Generation Partnership Project (3GPP). The specifications related to E-UTRAN are known as the 36 series specifications and are available for download from the 3GPP website at http://www.3gpp.org/DynaReport/36-series.htm. Several of the specifications include information helpful in understanding this disclosure, including: “LTE; Evolved Universal Terrestrial Radio Access (E-UTRA); LTE physical layer; General description,” 3GPP TS 36.201 version 12.1.0 Release 12, 2015-02; “LTE; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation,” 3GPP TS 36.211 version 12.4.0 Release 12, 2015-02; “LTE; Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2,” 3GPP TS 36.300 version 12.4.0 Release 12, 2015-02; and “LTE; Evolved Universal Terrestrial Radio Access (E-UTRA); Medium Access Control (MAC) protocol specification,” 3GPP TS 36.321 version 12.4.0 Release 12, 2015-02; all four of which are incorporated by reference herein.
In an E-UTRAN a base station is known as an enhanced node B (eNB) and a wireless terminal is known as user equipment (UE). An eNB can manage one or more cells, also called sectors, and is responsible for transmission and reception of wireless signals as well as interfacing with the core network, known as evolved packet core (EPC). It provides connectivity, that is, reliable data transfer (whenever possible), and control paths to the UEs in its coverage area. An eNB communicates with a UE using a pair of carrier frequencies, one for uplink (UL) and one for downlink (DL), if using Frequency Division Duplex (FDD), or using a single carrier frequency for both UL and DL if using Time Division Duplex (TDD). The DL uses Orthogonal Frequency Division Multiple Access (OFDMA) modulation of the carrier, and the UL uses Single-Carrier Frequency Division Multiple Access (SC-FDMA), which is also known as Linearly precoded OFDMA (LP-OFDMA). While the two modulation schemes are different, they share many similar qualities, and term “OFDM” may generally be used herein to describe either scheme.
In a traditional implementation, eNBs are separate logical entities, connected to the same core network via the S1 interface, which can be conveyed over a wired or wireless backhaul connection. An optional X2 interface may be used to directly connect neighbor eNBs. During a call, a UE may be handed over to neighbor eNBs, depending on radio conditions and traffic load. Handovers imply, among other things, a transfer of “context” of the UE being handed over, from a source eNB to a destination eNB. Such transfer may occur via the S1 interface or via the X2 interface, if available. The functions of the eNB can be broadly classified as radio frequency (RF) functions that deal with radio frequency signals, and baseband (BB) operations that deal with digital data.
eNBs implement several functions which together can be classified baseband (BB) operations. The baseband operations include the physical-layer (PHY) functions, medium access control (MAC) layer functions, radio link control (RLC) layer functions, packet data converge protocol (PDCP) layer functions, and radio resource control (RRC) layer functions.
Physical-layer (PHY) functions include, among others, transmission of downlink physical channels, such as physical downlink control channel (PDCCH), physical downlink shared channel (PDSCH), and cell-specific reference signal (CRS). The PHY layer functions also include reception of uplink physical layer channels, such as physical uplink shared channel (PUSCH), physical uplink control channel (PUCCH), physical random access channel (PRACH) and sounding reference signal (SRS). The PHY layer also handles synchronization, measurements of radio conditions, and other miscellaneous functions.
Medium access control (MAC) layer functions include scheduling, mapping of transport channels to logical channels, maintenance of uplink time alignment, hybrid automatic repeat request (HARD) operation, and discontinuous reception (DRX). Radio link control (RLC) layer functions include concatenation, segmentation, reassembly, reordering, and error correction (through ARQ). Packet data convergence protocol (PDCP) layer functions include in-sequence delivery of data units, header compression, elimination of duplicates, ciphering and deciphering, and integrity protection and verification. Radio resource control (RRC) layer functions include broadcast of system information, connection control, mobility, and measurement configuration and control.
In a traditional implementation, all eNB functions are carried out by specialized hardware devices, dedicated telecommunications equipment, data centers, and the like. In such traditional systems, the entire eNB is located in one location, allowing the eNB to be managed as a single unit. In another implementation, one or more eNBs are managed by the same hardware device or co-located devices and the transmission and reception antennas corresponding to the eNBs are distributed in a potentially large region. In such implementation, group of co-located antennas may be denoted as remote radio heads (RRHs), and a special interface connects the processing device with the RRHs it manages. A RRH can also be referred to as a remote radio unit (RRU) and the terms are used interchangeably herein.
In one implementation, which may be referred to as a distributed RAN or a Cloud-RAN, an RRH is targeted to have a smaller form factor, reduced power consumption, and lower operating expenses. To meet this goal, the RRH implements a minimum set of functions. In such implementations, the RRH may only include radio frequency (RF) functions such as amplification, filtering, up and down frequency conversion, digital to analog and analog to digital conversions, and the like, and baseband processing is still performed by dedicated equipment, which may not be co-located with the RRH.
A block diagram of a traditional distributed RAN 100 is shown in FIG. 1. The RAN 100 includes a central office 102 with one or more baseband units (BBUs) 160 that include all of the functionality for the PHY layer and the MAC layer of the RAN protocol. The RAN 100 also includes multiple RRUs 130 that include RF functionality and are each coupled to one or more antennas 131 to communicate with UE devices, such as smartphones. The interface between a BBU 160 and an RRH 130, is referred to as a fronthaul link 135. A traditional fronthaul link 135, can utilize a wired, optical, or radio frequency physical link, but the traditional fronthaul link is synchronous, low-jitter, and usually point-to-point. The fronthaul link 135 may be referred to as being “fiber-grade” indicating that it is high speed and low latency with minimal jitter. In some cases, the fronthaul link 135 uses a proprietary protocol, but in many implementations, a standardized protocol, such as the Common Public Radio Interface (CPRI) or the Open Base Station Architecture Initiative (OBSAI), is used. A central office 102 may host multiple BBUs 160, which in turn may control multiple RRUs 130. The BBUs 160 of the central office 102 are coupled to the core network, or EPC 199, by a backhaul link 190, that may utilize standard networking technology and is much more tolerant of latency and jitter issues than the fronthaul link 135.
One key issue with a distributed RAN 100 architecture is latency. Different functions in the baseband stack can have different requirements of end-to-end latency. As an example, HARQ, implemented in the MAC layer, requires an end-to-end delay of less than 4 ms in an LTE FDD (frequency division duplex) implementation. This means that from the time a UE transmits a data packet via the PUSCH channel, there is a maximum time, set by the specification, for the eNB to provide a corresponding HARQ response, e.g. a non-acknowledgement (NACK) to the UE. The overall latency for performing a specific function of the baseband stack, e.g. downlink (DL) HARQ processing, includes the time spent by the UE to perform the function, the bidirectional propagation delay over the wireless medium, and any propagation and processing delay over the fronthaul link 135 connecting the antennas and the BBU 160.
If the overall latency for performing a specific function does not satisfy specific constraints imposed by the standard for the function, the system may fail, and communication between an eNB and a UE cannot be sustained. Thus, latency constraints need to be satisfied in all operating conditions. This requires hard, real-time, constraints on the processing of specific latency-constrained functions, or functions that have an impact on the overall system timeline. Furthermore, fronthaul propagation delays from the antennas 131 to the BBUs 160 also must be accounted for within the overall latency constraint. In order to minimize the additional latency introduced by exchange of data between the BBUs 160 and the RRUs 130, a fiber-grade fronthaul is traditionally used.